The present invention relates generally to manufacturing processes, and, more specifically, to casting.
Investment casting or the lost-wax process is used for forming complex three dimensional (3-D) components of a suitable material such as metal. An exemplary cast component is the typical turbine rotor blade of gas turbine engine.
A turbine blade includes an airfoil integrally joined at its root with a blade platform below which is integrally joined a multilobed supporting dovetail. The airfoil is hollow and includes one or more radial channels extending along the span thereof that commence inside the blade dovetail which has one or more inlets for receiving pressurized cooling air during operation in the engine.
The airfoil may have various forms of intricate cooling circuits therein for tailoring cooling of the different portions of the opposite pressure and suction sides of the airfoil between the leading and trailing edges thereof and from the root at the platform to the radially outer tip.
Complex cooling circuits include a dedicated channel inside the airfoil along the leading edge for providing internal impingement cooling thereof. A dedicated channel along the thin trailing edge of the airfoil provides dedicated cooling thereof. And, a multi-pass serpentine channel may be disposed in the middle of the airfoil between the leading and trailing edges. The three cooling circuits of the airfoil have corresponding inlets extending through the blade dovetail for separately receiving pressurized cooling air.
The cooling channels inside the airfoil may include local features such as short turbulator ribs or pins for increasing the heat transfer between the heated sidewalls of the airfoil and the internal cooling air. The partitions or bridges which separate the radial channels of the airfoil may include small bypass holes therethrough such as the typical impingement cooling holes extending through the forward bridge of the airfoil for impingement cooling the inside of the leading edge during operation.
Such turbine blades are typically manufactured from high strength, superalloy metal materials in conventional casting processes. In the common investment casting or lost-wax casting process, a precision ceramic core is first manufactured to conform with the intricate cooling passages desired inside the turbine blade. A precision die or mold is also created which defines the precise 3-D external surface of the turbine blade including its airfoil, platform, and integral dovetail.
The ceramic core is assembled inside two die halves which form a space or void therebetween that defines the resulting metal portions of the blade. Wax is injected into the assembled dies to fill the void and surround the ceramic core encapsulated therein. The two die halves are split apart and removed from the molded wax. The molded wax has the precise configuration of the desired blade and is then coated with a ceramic material to form a surrounding ceramic shell.
The wax is melted and removed from the shell leaving a corresponding void or space between the ceramic shell and the internal ceramic core. Molten metal is then poured into the shell to fill the void therein and again encapsulate the ceramic core contained in the shell.
The molten metal is cooled and solidifies, and then the external shell and internal core are suitably removed leaving behind the desired metallic turbine blade in which the internal cooling passages are found.
The cast turbine blade may then undergo subsequent manufacturing process such as the drilling of suitable rows of film cooling holes through the sidewalls of the airfoil as desired for providing outlets for the internally channeled cooling air which then forms a protective cooling air film or blanket over the external surface of the airfoil during operation in the gas turbine engine.
Gas turbine engine efficiency is increased typically by increasing the temperature of the hot combustion gases generated during operation from which energy is extracted by the turbine blades. The turbine blades are formed of superalloy metals, such as nickel based superalloys, for their enhanced strength at high temperature to increase the durability and useful life of the turbine blades.
The intricate cooling circuits provided inside the airfoils are instrumental in protecting the blades from the hot combustion gases for the desired long life of the blades in an operating turbine engine.
The cooling circuits inside turbine blades are becoming more and more complex and intricate for tailoring the use of the limited pressurized cooling air and maximizing the cooling effectiveness thereof. Any such cooling air bled from the compressor during operation for cooling the turbine blades is not used in the combustion process and correspondingly decreases the overall efficiency of the engine.
Recent developments in improving turbine airfoil cooling include the introduction of double walls therein for enhancing local cooling of the airfoil where desired. The typical airfoil includes main channels such as the dedicated leading edge and trailing edge channels and the multi-pass serpentine such as the dedicated leading edge and trailing edge channels and the multi-pass serpentine channels that provide the primary cooling of the airfoil. These channels are typically defined between the thin pressure and suction sidewalls of the airfoil which may be about 40 to 50 mils thick.
In introducing double wall construction of the airfoil, a thin internal wall is provided between the main sidewalls of the airfoil and the main channels therein to define auxiliary or secondary channels which are relatively narrow. The secondary wall may include impingement holes therethrough for channeling from the main flow channels impingement cooling air against the inner surface of the main sidewalls.
The introduction of the double wall construction and the narrow secondary flow channels adds to the complexity of the already complex ceramic cores used in typical investment casting of turbine blades. Since the ceramic core identically matches the various internal voids in the airfoil which represent the various cooling channels and features thereof, it becomes correspondingly more complex as the cooling circuit increases in complexity.
Each radial channel of the airfoil requires a corresponding radial leg in the ceramic core, and the legs must be suitably interconnected or otherwise supported inside the two dies during the casting process. As the ceramic core legs become thinner, such as for the secondary channels, their strength correspondingly decreases which leads to a reduction in useful yield during the manufacture of the cores that are subject to brittle failure during handling.
Since the ceramic cores are separately manufactured and then assembled inside the two die halves, the relative positioning thereof is subject to corresponding assembly tolerances. The walls of the airfoil are relatively thin to begin with, and the features of the ceramic core are also small and precise. Therefore, the relative position of the ceramic core inside the die halves is subject to assembly tolerances which affect the final dimensions and relative position of the intricate cooling circuit inside the thin walls of the resulting airfoil.
Accordingly, it is desired to provide an improved casting method for 3-D components having intricate internal voids.